1. Field of the Invention
The present invention generally relates to echo cancellation for data transmission, and more particularly to a system and method for echo cancellation in an environment having an asymmetric spectrum, such as asymmetric digital subscriber line (ADSL), using either single-carrier or multi-tone modulation.
2. Discussion of the Related Art
Full-duplex data transmission is the transmission of data in opposite directions simultaneously. When full-duplex transmission uses overlapping frequency bands, echo is the undesired leakage of a transmitted signal into the co-located receiver, superimposed upon and thus interfering with the intended reception. As an example, in a telephone network, one type of echo occurs when a customer""s transmitted signal leaks through the hybrid circuit back into that customer""s receiver, interfering with the transmitted signal from another customer.
In a communication over wire, such as ADSL (asymmetric digital subscriber line), the data traveling in the two opposite directions use asymmetric but sometimes overlapping signal bandwidth. Even in cases where the in-band spectra in the two opposite directions do not overlap, the inevitable transition bands may still cause in-band echoes. An echo cancellation technique must then be employed to separate the upstream and downstream signals in the overlapping spectrum. The performance of an echo canceler is of essence since in many cases the echo is much stronger than the intended signal.
FIG. 1 is a block diagram of a prior art system, wherein the transmitted signals in the two opposite directions are mixed and separated at the hybrid connectors. Echo occurs primarily because of the impedance mismatch between the hybrid connector and the two-wire phone line. An echo canceler operates by first estimating the parameters of the echo path, and then convolving the estimate with the transmitted data, thus emulating the echo. This emulated echo (also referred to herein as echo replica) is then subtracted from the received signal, which ideally results in an echo-free transmission.
Because the impedance of the transmission lines is time varying as well as line dependent, the echo canceler must be adaptivexe2x80x94it must learn the echo characteristics and track time-varying changes. The echo canceler can be made adaptive as follows: After estimating the echo parameters, emulating the echo, and subtracting it from the received signal, the remaining signal (which contains some residual echo) is fed back and used to update the estimated set of echo parameters. This feedback loop allows the echo canceler to converge to a close approximation of the echo parameters.
Echo cancellation may be accomplished either in the time domain or in the frequency domain. In time-domain echo cancellation, echo parameters are derived by estimating the echo path impulse response, and echo emulation involves a time-domain convolution. In frequency-domain echo cancellation, the echo parameters are obtained by estimating the frequency response of the echo path rather than the impulse response of the echo path. Echo emulation and adaptive update can then take place in the frequency domain, using the estimate of the frequency response of the echo channel.
Multicarrier modulation is a means of transmitting data by first grouping the binary digital data to be transmitted into sub-blocks. The sub-blocks are then further grouped into fixed-length blocks. As an example, each sub-block may contain one to eight bits, and 100 sub-blocks will form a block. For transmission across the communication channel, a set of carrier signals is used whose frequencies lie at evenly spaced values across the usable frequency band. This set of carrier signals is equal in number to the number of sub-blocks within a block. Continuing the above example, with a useful bandwidth of 1.0 MHz, 100 carrier signals would be used at multiples of 10 kHz. The bits within each sub-block are then used to modulate the corresponding carrier signal. The set of modulated carriers are then added together, and the resultant signal is transmitted across the channel.
Most existing time-domain echo cancelers use the tapped-delay line structure (also known as an FIR filter) to model the echo path and replicate the echo. However, at high sampling rates, the FIR filter can be several hundred taps long in order to provide enough coverage for the entire echo path, and the computational complexity is high. A prior-art invention specifically related to discrete multi-tone (DMT) modulation alleviates this computational complexity by performing the bulk of echo emulation in the frequency-domain, but it requires a tangled receiver structure in which equalization is placed prior to echo cancellation, consequently a complicated synchronization procedure, and a large amount of memory.
Accordingly, an alternative solution that overcomes the shortcomings of the prior art is desired.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the advantages and novel features, the present invention is generally directed to method and apparatus for echo cancellation over an asymmetric transmission and receiving spectra, specifically when the transmitted spectrum is greater than that of reception. In essence, the system implements identical low-pass decimation circuits between the transmit path and an echo canceler and the receive path and the echo canceler, thereby reducing the sampling rate and the number of echo canceler coefficients without affecting coverage. The echo canceler may be implemented in accordance with conventional echo cancellation techniques. Advantageously the decimation circuits significantly reduce the memory requirement and thus computation of the echo canceler. Since the inventive method and apparatus operate strictly in the time domain, essentially no synchronization is required, and they can be readily applied to both single-carrier and multi-carrier transmission.
In accordance with one aspect of the invention, an apparatus is provided having a transmit path for communicating data within a first bandwidth, a receive path for communicating data within a second bandwidth that is smaller than said first bandwidth, and an adaptive echo canceler disposed between the transmit path and the receive path. A first decimator is disposed between the transmit path and the echo canceler circuit, whereby the first decimator filters an incoming signal having a first sampling rate on said transmit path and emits a signal output at a second, reduced sampling rate. A second decimator, identical to said first decimator, is disposed along said receive path whereby said second decimator filters an incoming signal having a sampling rate approximately equal to the first sampling rate, to emit a signal output at a reduced sampling rate, approximately equal to the second sampling rate. Finally, an adder is disposed to subtract the output of the adaptive echo canceler from output of the second decimator to generate a received signal that is substantially free of echo.
In accordance with the preferred embodiment the first and second decimators are identical in function and operation. Consequently the required coverage of the echo canceler, which translates directly into the number of coefficients, needs not to be extended, in spite of the effective insertion of the second decimator into the echo path. Preferably, each operates to decimate an input signal by a ratio of four to one for T1.413/G.dmt, or two to one for UAWG/G.lite, all of which discrete multi-tone (DMT) ADSL standards or pending standards. In the preferred embodiment, the transmit path includes a signal having a sampling rate of 2,208 kilo-samples per second and the receive path includes a signal having a sample rate of 552 kilo-samples per second.
In accordance with another aspect of the invention, a method is provided for echo cancellation in a system having a transmit bandwidth that is larger than the receive bandwidth. In accordance with this inventive aspect, the method includes the steps of decimating a transmission reference signal and inputting the decimated signal into an echo canceler, and decimating a received signal. It should be emphasized here that decimation, or bandwidth reduction, of the received signal can alternatively be implemented in the analog domain. Then, an output of the echo canceler is subtracted from an output of the decimated received signal. Finally, the echo residual, i.e., the result of the subtraction, is used to adapt the echo canceler.